This invention relates to a liquid crystal display devices for displaying an image, for instance, and more particularly, to an active matrix type liquid crystal display device that has a plurality of display electrodes provided in a liquid crystal cell and selectively driven by thin film transistors for display.
A prior art liquid crystal display device of the conventional type has a structure as shown in FIG. 1. The device comprises a pair of transparent substrates 11, 12 of glass or the like facing each other. A spacer 13 is provided between the substrates 11 and 12 along the edges thereof. A liquid crystal 14 is sealed between the substrates 11 and 12. The substrates 11 and 12, spacer 13 and liquid crystal 14 constitute a liquid crystal cell. A plurality of display electrodes 15 consisting of a transparent conductive film are formed on the inner surface of one of the transparent substrates, i.e., substrate 11. Also thin film transistors 16 are formed as switching elements such that they are contiguous to the display electrodes 15 with their drains connected thereto. A common electrode 17 is formed on the other transparent substrate 12. The common electrode 17 faces the plurality of display electrodes 15.
The display electrodes 15 serve as picture element electrodes, for instance. As shown in FIG. 2, they are square in shape and arranged in a closely spaced-apart relation to one another. They are arranged in rows and columns on the transparent substrate 11. Gate buses 18 are formed such that they extend near and along the individual rows of display electrodes 15. Source buses 19 are formed such that they extend near and along the inidivual columns of display electrodes 15. The thin film transistors 16 noted above are formed at the intersections of the gate buses 18 and source buses 19. Each thin film transistor 16 has its gate connected to the associated gate bus 18, its source connected to the associated source bus 19 and its drain connected to the corresponding display electrode 15.
When one of the gate buses 18 and one of the source buses 19 are selected, a voltage is applied between the selected buses. As a result, only the corresponding thin film transistor 16 is turned on. Charge is stored on the display electrode 15 connected to the drain of the "on" thin film transistor 16. A voltage is applied across only a portion of the liquid crystal 14 between this display electrode 15 and the common electrode 17. Only this display electrode 15 is thus rendered transparent or opaque. In this way, only selected display electrodes are driven for display. Actually, selected display electrodes are usually rendered transparent or opaque in combination with a polarizer (not shown).
Usually, the thin film transistor 16 has a structure as shown in FIGS. 3 and 4. Referring to these Figures, the display electrodes 15 and source buses 19 are formed from a transparent conductive film, e.g., an ITO, on the transparent substrate 11. A semiconductor layer 21 or amorphous silicon or the like is formed such that it strides parallel and closely spaced-apart portions of each display electrode 15 and the associated source bus 19. A gate insulating film 22 of silicon nitride or the like is formed on the semiconductor layer 21. A gate electrode 23 is formed on the gate insulating film 22 such that it overlies part of each display electrode 15 and associated source bus 19 via the gate insulating film 22 and each semiconductor layer 21. One end of the gate electrode 23 is connected to the associated gate bus 18. Portions of the display electrode 15 and source bus 19 facing each gate electrode 23 constitute drain and source electrodes 15a and 19a, respectively. The thin film transistor 16 is constituted by these electrodes 15a and 19a, the semiconductor layer 21, the gate insulating film 22 and the gate electrode 23. The individual gate electrode 23 and gate buses 18 are formed simultaneously from aluminum, for instance.
In the above prior art structure, the semiconductor layer 21 is formed only over the region where the thin film transistor 16 is formed. Therefore, there is a considerable lever difference between the gate electrode 23 and gate bus 18 with respect to the substrate 11. Due to this level difference, breakage of the connection between the gate electrode 23 and gate bus 18 is liable to result.
Accordingly, it has been proposed to form the semiconductor layer 21 and the gate insulating film 22 such that each of them has the same pattern as that of the gate electrode 23 and gate bus 18, as shown in FIG. 5. More specifically, in this proposal each of the semiconductor layer 21 and gate insulating film 22 is formed over the entire surface other than the regions of display electrode 15 and source bus 19 in the same pattern as that of the contiguous gate bus 18 and gate electrode 23 to be formed. Then an aluminum layer, for instance, for the gate bus 18 and gate electrode 23, is formed over the two layers. With this structure, the level difference between the gate bus 18 and gate electrode 23 with respect to the substrate 11 is reduced to reduce the possibility of breakage of connection. In this case, however, the surface distance (i.e. distance along the surface) between the gate electrode 23 and the source electrode 19a and that between the source electrode 19a and source bus 18 are reduced, leading to the problems of current leaks between these parts.
Further, with the structure noted above a parasitic thin film transistor is formed by a portion 21a (not shown in FIG. 5; see FIGS. 8 and 16B) of the semiconductor layer 21 that does not constitute the thin film transistor 16, gate insulating film 22, source bus 19 and drain electrode 15a. The amorphous silicon of the semiconductor layer 21 is photo-conductive, and its resistivity is reduced from 10.sup.9 .OMEGA.-CM to about 10.sup.4 .OMEGA.-cm when it is illuminated by light of 100,000 luxes. Therefore, when the semiconductor layer portion 21a of the parasitic thin film transistor is illuminated, a current leak from the source base 19 adjacent to the thin film transistor 16 is caused to deteriorate the on-off ratio thereof.
In order to prevent deterioration of the on-off ratio of the thin film transistor 16 with the illumination of the semiconductor layer 21 thereof, a light-blocking layer 25 of chromium or like metal is formed such that it faces the semiconductor layer 21, as shown in FIG. 5. In this case, the light-blocking layer 25 is insulated from display electrode 15 and source bus 19 by an insulating layer 27. It may be thought to form the light-blocking layer 25 such that it faces the entire surface of the semiconductor layer portion 21a to avoid the influence of the parasitic thin film transistor noted above. In this case, however, the electrostatic capacitance between the light-blocking layer and gate bus is increased too much to permit high speed control of the thin film transistor.
To reduce the electrostatic capacitance between the light-blocking layer 25 and drain and source electrodes 15a and 19a and that between the afore-mentioned extension of the light-blocking layer 25 and gate bus, it may be thought to increase the thickness of the insulating layer 27. Doing so, however, requires increased time for manufacture and deteriorates the yield.
In a further aspect, the source bus terminal is constituted by the same material as the source bus, while the gate bus terminal is constituted by the same material as the gate bus. For example, the source bus terminal is made of a transparent conductive material, e.g., ITO, while the gate bus terminal is made of aluminum. Since the two terminals are made of different materials, their connection to an external drive circuit requires different connection steps suited to these materials. The connection process, therefore, is rather cumbersome.
Further, the ITO constituting the source bus 19 has a comparatively low electric conductivity, that is, the source bus 19 has a comparatively high electric resistance. Therefore, a comparatively large difference in the drive voltage is produced between the opposite ends of the source bus 19. In other words, the transistors provided along the source bus can not be driven with a uniform voltage. This leads to a brightness slope in the screen of the liquid crystal display device according to the potential gradient along the source bus. It may be thought to increase the thickness of the source bus 19 to solve this problem. Doing so, however, would result in the precipitation of In during the formation of ITO by the plasma chemical vapor deposition process. The ITO film is thus denatured to deteriorate the characteristics of the thin film transistor. Further, since the source bus 19 is transparent, light leaks from portions of the source bus 19 which are not overlaid by the transistors 16 and gate buses 18. This deteriorates the contrast of the displayed image.